Stub class for all Level set variants. SparseFieldLevelSet is an implementation of the sparse field method.
If there is some more efficient method in future, just replace the superclass.
The current superclass requires two functions, the updateDeltaT and the getDeltaPhi methods which differ in
the sub-classed implementations.
Before iterations, init() is called, after run cleanup() is called - implementations can add their own stuff there.